Pneumococcus (Streptococcus pneumoniae) is one of the most frequently found pathogenic bacteria for community-acquired pneumonia and lower respiratory tract infections, showing high morbidity and mortality in the world, including Japan. Since infections caused by pneumococcus occur at high incidence rate and tend to become severe, selection of an appropriate antimicrobial drug is a key to the treatment at the start thereof. According to the principle of treatment of infections, determination of the pathogenic bacterium at as early a stage as possible is also important, since an appropriate remedy can be employed at an early stage, leading to improvement in prognosis, reduction in medical cost, and prevention of generation of resistant bacteria. Under such circumstances, there is demand for a diagnostic drug for rapidly detecting a pneumococcus-derived antigen at an early stage of infection.
Sorensen et al. have reported the structure of pneumococcus in detail (Non-Patent Document 1). The outermost surface of the cell is formed of a capsular, and a polysaccharide antigen, called capsular polysaccharide, is ligated to the capsular. Hitherto, some tens or more of serum types are known in accordance with various structures of capsular polysaccharides. Meanwhile, the pneumococcus has a cell wall inside the capsular and a plasma membrane inside the cell wall. C-polysaccharide (C-ps) is ligated to the cell wall, and teichoic acid or lipoteichoic acid, called an “F-antigen,” is ligated to the plasma membrane. C-ps is known to be a common antigen maintained in all capsular types of pneumococcus species, and the polysaccharide moiety of the F-antigen is known to have the same saccharide sequence as that of C-ps.
Hitherto, there have been known methods for detecting a pneumococcal antigen through employment of immunoassay. Examples of such methods include a detection method in which a C-ps antigen in a sputum sample is detected through ELISA employing an anti-C-ps antibody (Non-Patent Documents 2, 3), and a detection method in which a capsular polysaccharide (antigen) in a serum sample or a urine sample is detected through immuno-electrophoresis (Non-Patent Documents 4, 5).
Conventionally, there have been used pneumococcus detection kits based on the latex agglutination method, which kits detect a pneumococcal antigen in a cerebrospinal fluid sample, a serum sample, or a urine sample. The principle of the kits is thought to be based on detection of a polysaccharide moiety of capsular polysaccharide or the like (Non-Patent Documents 6, 7). At present, however, substantially no kit based on the latex agglutination method is employed due to cumbersome operation and unsatisfactory sensitivity.
Currently, simpler detection means are employed. One of such means is a speedy detection kit for an antigen in urine (Binax NOW (registered trademark), Streptococcus pneumoniae urinary antigen test, product of Binax Inc.). In this kit, C-ps present in a urine sample is detected through immunochromatography (Patent Document 1). This method is non-invasive, since an antigen in a urine sample is detected. The time required for the detection is as short as about 15 minutes (Non-Patent Document 8). However, this method (detection of antigen in urine) has a drawback in that a false positive result may be obtained due to continuous excretion of pneumococcus for a long period of time after termination of the relevant therapy (Non-Patent Document 9). In addition, difficulty is encountered in collecting urine from infants, and a false positive result may be obtained from the influence of an indigenous pneumococcus (Non-Patent Document 10). The kit is thought to provide slightly low sensitivity (Non-Patent Document 11).
A more recently developed kit; i.e., a pneumococcal antigen detection kit (Non-Patent Document 12), rapidly detects a pneumococcal antigen (C-ps) in a sputum sample, a swab of the nasal cavity, a swab of the epipharynx, a middle ear fluid sample, or an otorrhea fluid, through immunochromatography employing an anti-C-ps-polyclonal antibody (rabbit). As compared with the aforementioned diagnosis kit for an antigen in urine, the recently developed kit attains high sensitivity, and allows a clinical specimen (e.g., swab) to be analyzed without performing a concentration operation in advance. Furthermore, the detection of the antigen from a middle ear fluid sample, which was previously difficult, can be realized. Unlike a urine specimen, a sample can be easily obtained from infants. However, when certain samples such as those derived from the middle ears and the paranasal sinuses are analyzed by the kit, the sensitivity is still unsatisfactory. Thus, there is demand for such an antigen detection method attaining higher sensitivity.
As described above, there have already been reported pneumococcal antigen detection kits which detect capsular antigens or C-ps. Of these two type of kits, kits that detect capsular antigens require provision of various antibodies corresponding to various capsular antigens, and therefore, such kits are not useful as a simple assay tool. Regarding the second type; i.e., kits detecting C-ps, further enhancement in sensitivity is required in consideration of the performance test results of the aforementioned existing kits.
In contrast, the F-antigen has not been employed in routine clinical tests. Although there have been disclosed some immunological detection methods in which an anti-F-antigen antibody is employed, to detect a bacterium containing a pneumococcal antigen (Non-Patent Documents 13 to 15), in these methods, cross-reaction may occur between different bacteria, and a false negative result may be obtained, causing problematic assay accuracy.